From Lobster's Eyes to Alien Oceans

Tuesday 19th November, 2013

Many of the professional astronomers that we invite to talk to the Society became interested in the field in their younger days and Dr Nigel Bannister from Leicester University is no exception. In his youth he owned a 14-inch Dobsonian telescope to gaze at the stars and this has been upgraded nowadays to a 6-inch Takahashi refractor.

He currently works as a "system radiation designer" which means working out how to stop the instruments sent into space from being destroyed by a barrage of radiation. His latest project involves the design of a magnetometer, named J-MAG, aboard the European Space Agency's JUICE mission to Jupiter.

In his talk entitled "From Lobster's Eyes to Alien Oceans" Dr Bannister explained that the JUICE spacecraft is the first of a class of missions in ESA's Cosmic Vision programme spanning from 2015 to 2025. It is planned to launch in 2022 and on arrival at Jupiter in 2030 to spend at least three years observing Jupiter and three of its largest moons — Ganymede, Callisto and Europa.

To reach Jupiter it will perform a number of flybys of Earth and Venus, each time using the planet's gravity and motion in order to alter its course and speed. These manoeuvres actually slow down the planet's rotation as the spacecraft effectively "steals" some of the planet's orbital energy but the effect is so tiny the difference is not noticeable.

This "slingshot" technique was discovered by the American mathematician Michael Minovitch in the early 1960s when he was working at the Jet Propulsion Laboratory as a graduate student. The first mission to use the technique was the American space probe called Mariner 10 in 1973 which flew by Mercury and Venus.

When JUICE (JUpiter ICy moons Explorer) reaches Jupiter it will use the same "slingshot" technique but in reverse in order to slow the spacecraft down to settle into an orbit within the Jupiter system. This deceleration phase will occur around Ganymede which is the largest moon in our solar system. Dr Bannister described this moon as the archetype of an icy moon, having what is thought to be a liquid saltwater ocean between its icy layers. It is of special interest to scientists as it is the only other body in the solar system, apart from Mercury and Earth, to have a global magnetic field.

Apart from studying the main planet, Jupiter, JUICE will also make observations of the next largest Jovian moon, Callisto. This also has some puzzling features that the scientists would like to understand. For example, unlike Ganymede, it has roughly the same consistency throughout and its entire surface is covered in craters.

The spacecraft will also make some tentative flybys of Europa but as this moon is well within the hazardous radiation belts of Jupiter there are no plans to orbit it in case the radiation dose the spacecraft receives actually ends the mission prematurely. Mission planners prefer to deliberately control the end of the mission by de-orbiting JUICE so that it crashes into Ganymede's surface and hence protecting the subsurface oceans within these icy moons.